SAN DIEGO (AP) — A former Internal Revenue Service agent whose tax preparation business catered to a wealthy clientele is accused of ordering at least two former customers killed as they prepared to testify against him on fraud charges.

Federal prosecutors say the targets were key witnesses against Steven Martinez, 50, who was charged last year with stealing $11 million by preparing bogus tax returns for his customers.

Martinez’s limousine driver — Norman Russell Thellmann, 64 — was charged Monday with conspiracy to tamper with witnesses. Prosecutors allege he was ordered to deliver money to a hit man who was promised $100,000 for the two killings.

Martinez did not enter a plea during his initial court appearance Monday on a charge of witness tampering. A federal magistrate judge ordered him held without bail.

“I find it almost impossible to believe,” said David Demergian, his attorney.

Martinez, an IRS agent from 1988 to 1992, faces a pretrial hearing March 19 on federal fraud charges and was free on bail until his arrest last week. An FBI agent’s affidavit says Martinez gave a former employee documents on four people about two weeks ago, including photos of one target from the wealthy suburb of Rancho Santa Fe and another target’s condominium in the upscale La Jolla area of San Diego.

Martinez recommended the former employee use two different pistols for the killings and get a silencer, according to the affidavit. The former employee contacted the FBI, which recorded a meeting Thursday in which Martinez allegedly gave additional instructions like how to break into the La Jolla condominium.

The targets were identified as 86-year-old Monique Siegel of La Jolla and Marianne Harmon of Rancho Santa Fe.

The fraud complaint alleges that Martinez told customers to deposit their taxes into one of his bank accounts, promising to forward the money to state and federal authorities. He stated lower income on their tax returns without telling them, allowing him to pocket $11 million.

The complaint identifies victims only by their initials. One “M.H.” had an income of $20.7 million in 2006 but Martinez filed a tax return for $2.1 million. One “M.S.” earned $200,046 in 2006 but Martinez’s return reported $32,900.

Another customer who earned $12.2 million in 2005 reported income at $1.6 million, according to the complaint. The same customer earned $11 million in 2006, also reported as $1.6 million.

Demergian, his attorney, said the fraud case was “certainly very defensible.”

“He had a very dedicated loyal clientele,” Demergian said. “He was very successful.”

Thellmann, who was arrested Friday night, told the FBI that Martinez sold him a limousine about three years ago and hired him as a chauffeur. He said Martinez told him to give $40,000 to a person who would call him with code.

Thellmann denied he knew the money was to pay a hitman. FBI agents found $42,400 cash in a cereal box at his home.